1. Field of the Invention
This invention relates to a cylinder control device used in an industrial vehicle such as a forklift.
2. Description of the Related Art
A cylinder control device of this type is described, for instance, in Japanese Examined Utility Model Publication No.7-3043. The cylinder control device having downward motion preventing function as disclosed in that publication is designed to control a lift cylinder of a forklift, and provided with an operation check valve in a main passage communicating a bottom chamber of the lift cylinder with a lift cylinder operating spool valve so as to only permit communication from the spool valve side to the lift cylinder side. A pilot passage is provided with an electromagnetic on/off valve for an opening operation of the operation check valve so that the operation check valve is not opened unless the electromagnetic on/off valve is energized to be open. Further, in a state where the electromagnetic on/off valve is not energized, the lift cylinder is prevented from being moved downward even if the spool valve is erroneously operated.
The cylinder control device described in the above publication is designed so that the working oil of the lift cylinder, at the time of the downward movement of the fork, flows out through two passages, namely, the main passage for communication from the operation check valve through the spool valve to a tank, and another passage for communication from the pilot chamber of the operation check valve through the pilot passage of the operation check valve, the electromagnetic on/off valve and the spool valve to the tank.
For this reason, even if the electromagnetic on/off valve is fixed to or malfunctioned toward the xe2x80x9copenxe2x80x9d state, the upward and downward movements of the lift cylinder can be conducted without any abnormal feeling and similarly to the normal operation. Consequently, it is difficult for an operator to recognize an operation error of the electromagnetic on/off valve, and the electromagnetic valve may be left out of order. This gives rise to a problem in that the downward motion preventing function for the lift cylinder does not serve properly.
Further, the working oil, at the time of downward movement of the lift cylinder, flows out to the tank through two locations (two passages). This means that there are two locations or passages that must be processed with high precision. In other words, there are two locations or passages which are particularly relevant in determining a speed at the time of low-speed operation, and which must be finely adjusted to obtain proper opening degrees. Furthermore, portions of the spool valve and the valve body to be processed are also increased inevitably in number. Consequently, the manufacturing cost becomes high.
The cylinder control device constructed to have the operation check valve and the electromagnetic on/off valve as mentioned above is effective in view of enhanced safety since in an off-state of a key switch, the electromagnetic on/off valve is in a xe2x80x9cclosedxe2x80x9d state so that the lift cylinder is prevented from being moved downwardly even if the spool valve is erroneously operated. On the other hand, this, however, requires that a condition for opening the operation check valve with energizing of the electromagnetic on/off valve (the communication of the pilot chamber of the operation check valve with the tank passage) must be met in addition to the operation of the spool valve in order to enable the downward movement of the lift cylinder.
Therefore, in a case where the electromagnetic on/off valve can not be energized because of trouble, or the electromagnetic on/off valve can not be moved due to fixation, the electromagnetic on/off valve is kept closed, so that the operation check valve is not opened even if the spool valve is operated to be at the downward movement position, and accordingly the fork can not be moved downwardly.
In view of these problems, such a cylinder control device is proposed, that enables the downward movement of the lift cylinder even in a abnormal state where the electromagnetic on/off valve is fixed or cannot be energized. The cylinder control device of this type is disclosed, for instance, in a Japanese Utility Model Application Laid-open No. 1-91103.
The cylinder control device described in the above publication has such an arrangement in that the pilot chamber of the operation check valve is connected to a bypass passage communicated with the tank passage regardless of the opening/closing of the electromagnetic on/off valve, and a manually operable opening/closing valve is provided to the bypass passage.
Although the cylinder control device as mentioned above makes it possible to move the lift cylinder downwardly by operatively opening the opening/closing valve in the case of the abnormal state where the electromagnetic on/off valve is fixedly kept closed or can not be energized, the operation for the downward movement of the lift cylinder may cause a danger since the lift cylinder starts its downward movement immediately after the opening/closing valve is opened and it is difficult to adjust the speed of the downward movement.
The present invention has been made in view of the problems mentioned above. An object of the present invention is to provide a cylinder control device that can allow an operator to recognize an abnormality of an electromagnetic on/off valve and that can be manufactured with reduced cost.
Another object of the present invention is to provide a cylinder control device that enables a downward movement of a lift cylinder in an abnormal state where the electromagnetic on/off valve is fixedly kept closed or cannot be energized as safely as in the normal state.
A cylinder control device according to the present invention includes a main passage one end of which is communicated through a port with a cylinder, a spool valve for communicating another end of the main passage with a pump or a tank by switching operation, an operation check valve disposed on the main passage for permitting only the communication from the spool valve to the cylinder, the operation check valve having a pilot chamber communicated through a first orifice with a cylinder side of the main passage, a pilot passage one end of which is communicated with the pilot chamber and another end of which is communicated with a spool valve side of the main passage, and an electromagnetic valve for opening and closing the pilot passage.